In these years, optical discs such as CDs and DVDs are widely used in a broad range of applications such as the distribution of music, moving images, or software, the backup of a hard disk, digital video recording, and so on. Further increase in recording density in correspondence to further longer video recording time, smaller discs, and the spread of high-vision video, is desired and expected for the future. Examples of fundamental techniques for improving recording densities of optical discs include shortening the wavelength of the recording light source and increasing the resolution of the optical head lens, to mention two examples. For the next generation optical discs, recording capacities of about 25 GB per layer for 12 cm diameter discs and about 50 GB for double recording layer discs can be secured since a blue-violet laser (wavelength; 400 nm) is adopted as the recording light source. As means for further increase in recording density, a multilayer technology that increases the total number of recording layers of optical discs is attracting attention. This multilayer technology is a technology specific to optical discs that uses their transmissiveness to light and is a technology that selectively performs recording/reproduction by placing focus on an arbitrary recording layer while moving the focal position of the light spot in a direction of optical axis.
Patent literature 1 describes a multilayer optical disc apparatus which, while tracking based on tracking fixed information provided in a recording surface of an optical recording medium having multiple recording surfaces, records/reproduces data onto/from a recording surface other than the recording surface provided with the fixed information, and thereafter while tracking with the data as tracking information, records/reproduces data onto/from a recording surface other than the recording surfaces.
Patent literature 2 describes an information recording/reproducing apparatus which, while focusing a first light beam on a guide track layer of an information recording medium comprising a plurality of recording regions each having multiple recording layers and a plurality of guide track layers respectively for the plurality of recording regions, focuses a second light beam on any recording layer in the corresponding recording region to record/reproduce information.
Patent literature 3 describes a recording/reproducing method which, when recording/reproducing a recording medium having formed therein multiple recording layers, in only one of which a guide groove is provided, focuses a first light beam on the guide groove to track that recording layer, thereby performing the tracking of a second light beam simultaneously projected on another recording layer.    Patent literature 1: Japanese Patent Publication No. 2835074    Patent literature 2: Japanese Patent Kokai No. 2001-307344    Patent literature 3: Japanese Patent Kokai No. 2004-241088